Several mass-flow meters operate by producing characteristics variations in the fluid, or pulses, that are subsequently detected downstream from the point of creation to derive a flow rate. Mass-flow-meters which utilize this arrangement are generally referred to as Pulse Time-Of-Flight flow meters.
One such approach described in U.S. Pat. No. 4,532,811 to Miller, Jr. et al. applies a thermal pulse to a stream of fluid and has a single downstream heat sensor to sense the thermal pulse. The transit time between the heating element and the heat sensor determines flow velocity. The Miller thermal pulse technique is effective over a wide range of fluid temperatures, because the unheated fluid is used as a reference. The downstream sensor detects thermal pulses, i.e. envelopes of fluid traveling through the flow conduit that are warmer than the unheated fluid. Therefore, the thermal pulse technique is advantageously insensitive to changes in ambient temperature.
A major disadvantage of Miller's approach is measurement error associated with the transfer of heat to and from the fluid, and the transient time it takes the mark to integrate into the developed flow profile. Because the flow rate of the fluid at the edge of the flight conduit it less than the flow rate at the center of the conduit, the mark does not reach its equilibrium flow rate until its average position has moved to the center of the flow conduit. This rate of equilibration is associated with the thermal masses, thermal conductivities, heat-transfer coefficients of the heating element, sensor and fluid, and the viscosity and density of the fluid (i.e. Reynolds number), and must be accounted for when calculating flow rates. Since the delay is related to the properties of the fluid, the flow meter inconveniently must be recalibrated for different types and concentrations of fluids. Miller's approach assumes the flow rate from two time events associates with two identified positions of the mark as it conveys downstream with the flow, from which a single velocity of the mark can be derived and therefore a single flow rate can be derived per each mark detected. Miller's approach therefore can not extract additional information which is essential for deriving a more accurate determination of the flow rate. For instance, Miller's approach can not derive if the detected instant was during stable, accelerating, or decelerating speed. Such information could be beneficial to analyze the flow rate nature and to improve the conclusion of the average flow.
U.S. Pat. No. 5,533,412 to Jerman, et al. improves upon Miller's approach by providing at least two spaced apart sensors located along the flow conduit, downstream from the thermal marking position and the flow velocity is derived from the transient time it takes the pulse to travel between two sensors. The advantage of Jerman's approach is that it overcomes most of the fluid properties affects which degraded the accuracy in Miller's approach.
One significant drawback with the Jerman and Miller approaches is that it is inconvenient to have multiple detectors spaced downstream from the thermal marking position. Because it is preferred to use optical detection to note the arrival of the mark, it can become prohibitively expensive to use multiple detectors, as each additional diode and detector combination adds costs, and will require multiple alignments and calibrations of the optical components. As in Miller's approach Jerman's approach derives the flow rate from two time events for each mark and therefore has limited capacity of accurately concluding the average flow.
U.S. Pat. No. 6,660,675 to Mosier et al and continuation in part U.S. Pat. No. 7,225,683 to Harnett et al disclose a device for measuring over a wide range of flow rates which operates by marking the fluid by producing compositional variations in the fluid, or pulses, that are subsequently detected downstream from the marking position to derive a flow rate. Each pulse, comprising a small fluid volume, whose composition is different from the mean composition of the fluid, can be created by electrochemical means, such as by electrolysis of a solvent, electrolysis of a dissolved species, or electrodialysis of a dissolved ionic species. Measurements of the conductivity of the fluid can be used to detect the arrival time of the pulses, from which the fluid flow rate can be determined. A pair of spaced apart electrodes can be used to produce the electrochemical pulse mark.
The above listed prior art approaches suggest manufacturing techniques in a silicon chip made by micro-etching a silicon substrate and film deposition techniques i.e. using semiconductor micromachining technology.
Several patents disclose means for extending the dynamic range of the flow sensor U.S. Pat. No. 5,533,412 to Jerman, et al. discloses a flow sensor where velocity is measured in two portions of the channel, the portions having different cross-sectional areas, thereby providing different flow velocities. The narrower channel portion is used for measuring low flows, and the wide channel portion is used for measuring higher flows. This combines the dynamic ranges of the two portions, thereby substantially increasing the overall dynamic range of flow meter. Mosier and Harnett respectively disclose a three electrode configuration in which multiple pulse generators are spaced at unequal distances from the sense electrodes: the smaller in distance serves for measuring lower flow rates and the larger in distance serves to measure higher flow rates. This configuration advantageously increases the potential dynamic range of the sensor chip, but it disadvantageously requires the user to choose a write electrode (and its associated range) before use, and still only uses two time events to calculate the flight time of the marker.
It is a common practice in water metering applications and industrial flow metering applications to extend the dynamic range of the measurable flow rates by installing two or more flow meters, having a different dynamic range, in parallel. Preferably said water meters have a slight overlap in their dynamic range. At least one valve is set in the system to direct the flow to the lower flow range meter when the flow is not exceeding that range.
Prior art Time-Of-Flight techniques suffer from the following main disadvantages that effect the measurement accuracy and will be described herein:
In the prior art approaches it is assumed that if no pulse has been detected by the sensor then there is no flow (or zero flow). This approach can be referred to as negative detection. The disadvantage of negative detection is that it cannot differentiate lack of performance (such as out of measurable range flow rates) or malfunction of the meter where the pulse had not been detected for other reasons rather than zero flow (i.e. false negative detection). In other words, as some times referred to in this art, the meters in the prior art cannot differentiate between false-negative and true-negative. The reliability and accuracy of sensing zero flow is in several applications as significant or even more important than the accurate reading of real flow. For example in medical drug delivery applications it will be extremely important to prevent a situation where drug is being administered when it shouldn't or where it can not be monitored. In another drug delivery example a reliable (preferably positive) sensing of zero flow is important for detecting occlusion of the administration device or kink in the tube set of the delivery system etc. Mistakes in reading zero flow may result in serious error leading to hazardous situations or even a cause of death.
Also, prior art TOF MFM cannot distinguish between air bubbles and zero flow and therefore may introduce errors due to the presence of bubbles, or any other substantial change in the properties of the fluid. Another disadvantage of the prior art is that errors caused by diffusion (fading) of the pulse (mark) in the flow are ignored. The marker, be it thermal or composition variation, disposed in the flow has a tendency to diffuse and haze in the fluid. This tendency become significant at very low flows where the linear diffusion rates are competitive with the actual flow rate. The diffusion rate depends on the fluid properties and needs to be calibrated in currently disclosed methods. None of the known prior art Time-of-Flight flow meters provides a solution to the significant error that diffusion will cause at very low flows.
Another disadvantage of Time-Of-Flight measurement techniques proposed in the prior art is that they are sensitive to errors caused by sharp variations in the flow. The prior art techniques disclose means for calculating the flow based on two time events recorded for each mark. These time events may be the time of introducing the mark to the flow and the time that the mark reached a first sensor (Miller, Harnett, Mosier). Alternatively the two time events are proposed to be the time that the mark was detected by a first sensor and the time that the mark was detected by a second sensor spaced apart from the first sensor along the flight conduit (Jerman). The flow rate is calculated as an average flow rate over the transient time between two events, which may translate into significant errors if the flow is experiencing rate changes or if the base flow is low relative to noises (vibration) in the flow. Yet in various settings the flow may experience sharp variations in rate, which will affect the accuracy of the reading. For example if the meter is located in proximity with a pulsating pump the reading can give a wrong average if the measuring cycle is similar in length to numerous pulsation cycles. Another example of sharply varying flow is a drug delivery tube set where impacts on a relatively flexible tube set create pressure waves in the flow leading to vibrations in the flow rate. In another example of unstable flow, a pump is activated for a very short on-cycle and returns to an off cycle immediately thereafter, creating a sharp flow pulse which does not have a momentary stable flow. Therefore any attempt to conclude an average of the flow rates from two time events will most likely result in extreme errors.
It is therefore an object of the present invention to provide means for improving the accuracy of prior art Time-Of-Flight mass flow metering techniques by correcting for the diffusion rates of the pulse at very low flows.
It is another object of the present invention to provide improvement to prior art Time-Of-Flight mass flow metering techniques by reducing errors due to varying flow by calculating flow rates from at least three time events of the mark.
It is another object of the present invention to provide improvement to prior art Time-Of-Flight mass flow metering techniques by reducing errors due to diffusion by incorporating data from at least three time events. Such an improvement can include the use of an algorithm that incorporates at least a first derivative of the flow rate. The multiple readings of a mark support an algorithm incorporating a complex model of fluid flow, in order to fit the equations of flow.
It is yet another object of the present invention to provide means for positively detecting zero flow.
It is another object of the present invention to detect air bubbles and eliminate flow reading errors due to the presence of air bubbles.
It is yet another object of the present invention to integrate other sensors and devices with the meter platform including sensors for measuring or monitoring temperature, pressure, pH, free oxygen, viscosity, G-shock.
It is yet another object of the present invention to provide means for detecting viscosity of the fluid and variation in viscosity, and correct flow measurement errors.
It is another object of the present invention to provide means to detect fluid base conductivity and variations in the fluid base conductivity and provide indications of the fluid properties. In particular in medical application to alarm if there's suspect that the wrong fluid is administered to a patient.